1. Field of the Invention
The present invention relates to an optical scanning unit that scans a beam emitted by a light source onto an exposed object. More particularly, the present invention relates to an optical scanning unit designed to provide sufficient optical path length between a beam deflector and an exposed object while reducing the space taken up by the overall unit.
2. Description of the Related Art
Typically, an optical scanning unit is used for a laser printer, a digital copier, a facsimile, or the like to produce a latent image on an exposed object through main scanning by a beam deflector and subscanning by rotation of the exposed object.
Referring to FIGS. 1-3, a conventional optical scanning unit includes a light source 1 that produces and emits a beam, a beam deflector 7 that deflects the beam emitted by the light source 1 to be scanned onto an exposed object 15 in the main scanning direction, and an f-θ lens 11 that corrects an error contained in the beam deflected by the beam deflector 7. In addition, a collimating lens 3 that collects the divergent beam from the light source 1 and a cylindrical lens 5 that shapes the beam are located in an optical path between the light source 1 and the beam deflector 7, and a mirror member 13 that changes the direction of the scanned beam may be disposed between the f-θ lens 11 and the exposed object 15.
The beam deflector 7 consists of a driving source 9 and a rotating polygonal mirror 8 rotated by the driving source 9. The direction in which a beam emitted by the light source 1 is scanned is determined according to the reflection direction of the beam, which changes as the rotating polygonal mirror 8 rotates. Thus, when the rotating polygonal mirror 8 rotates clockwise, the beam incident on the rotating polygonal mirror 8 is scanned onto the exposed object 15 in the main scanning direction as indicated by an arrow.
The f-θ lens 11 is comprised of two lenses 11a and 11b. While one lens 11a compensates for any astigmatism present in the beam deflected by the beam deflector 7, the other lens 11b focuses the scanned beam and corrects the beam so that scanning lines are equiangular and equally spaced.
The optical scanning unit configured above has to provide an optical path D1 of sufficient length between the light source 1 and the beam deflector 7 since multiple optical components are arranged between them. Furthermore, it must provide an optical path D2 of sufficient length between the beam deflector 7 and the exposed object 15 since the beam deflected by the beam deflector 7 is incident on the entire main scanning area of the exposed object 15. However, since the overall size of the optical scanning unit increases in order to satisfy the above requirements, there is an insufficient space to install it when used in printing equipment and the main body of the printing equipment may become bulky.